1. Field of the Invention
The present invention relates generally to a glass composition that has high heat resistance and can be strengthened to a considerably higher degree by a chemical strengthening treatment involving an ion exchange, and a glass article including the glass composition, particularly a glass substrate useful for magnetic recording media. Furthermore, the present invention also relates to a method of producing a glass sheet that has high heat resistance and can have high mechanical strength provided by a chemical strengthening treatment involving an ion exchange.
2. Related Background Art
Glass has excellent properties such as, for instance, high surface smoothness and high surface hardness. Hence, glass is suitable for substrates for information recording media to be used in, for example, hard disk drives (magnetic recording devices; hereinafter referred to as “HDD”).
Glass, however, has the disadvantage that it is easily broken or cracked. As a countermeasure against this, conventionally, a compression stress is given to the surface of glass through quenching or an ion exchange, that is to say, a so-called strengthening treatment is conducted. Of such strengthening treatments, a chemical strengthening treatment that is conducted through an ion exchange is suitable for materials to be used for substrates requiring particularly high dimensional accuracy because it causes much less glass deformation during the strengthening as compared to other strengthening treatments.
Recently, there is a trend to increase further the recording density of information recording media. In order to achieve a higher recording density, a magnetic material to be formed on a substrate is provided with an increasingly complicated layer structure. Furthermore, in order to provide the magnetic material with advanced characteristics, when a magnetic material layer is formed on a substrate, the substrate must be heated to a high temperature. A perpendicular magnetic recording system has been expected to be mainstream for the information recording method from now on. However, when a magnetic material for perpendicular magnetic recording is formed, the substrate must be heated to a considerably higher temperature (400° C.) than that conventionally employed.
JP2837134B discloses a glass substrate for information recording that is characterized in being formed of a chemically strengthened glass obtained through an ion-exchange treatment of a glass intended to be chemically strengthened. The ion-exchange treatment is carried out in a treatment bath containing Na ions and/or K ions. The glass intended to be chemically strengthened contains, in terms of weight percentage, 62% to 75% of SiO2, 5% to 15% of Al2O3, 4% to 10% of Li2O, 4% to 12% of Na2O, and 5.5% to 15% of ZrO2, wherein a weight ratio of Na2O/ZrO2 is 0.5 to 2.0 and a weight ratio of Al2O3/ZrO2 is 0.4 to 2.5.
JP9-2836A discloses a glass substrate for magnetic disks obtained through a chemical strengthening treatment of a glass. The glass has a composition that essentially includes, in terms of weight percentage, 50 to 65 of SiO2, 5 to 15 of Al2O3, 2 to 7 of Na2O, 4 to 9 of K2O, 12 to 25 of MgO+CaO+SrO+BaO, and 1 to 6 of ZrO2, wherein the sum of Na2O and K2O is 7 to 14.
In recent HDDs, a magnetic recording medium is rotated at high speed and thereby its substrate is subjected to a great centrifugal force. Furthermore, it is necessary for a magnetic recording medium or a substrate for magnetic recording media to be able to withstand satisfactorily the impact that is caused by the collision of a recording head therewith when the magnetic recording medium or the substrate for magnetic recording media is included in a HDD that is operated with a so-called load-unload system (a ramp-load system). In this system a recording head is placed in a stand-by position while a magnetic recording medium is not rotated and the recording head is loaded thereon when it starts rotating. Accordingly, recent magnetic recording media are required to have higher strength than that of conventional magnetic recording media.